1. Using current- and voltage-clamp techniques, we examined the biophysical properties of a voltage-dependent Ca2+ current and its physiological role in plateau potential generation in the dorsal gastric (DG) motor neuron of the stomatogastric ganglion in the crab, Cancer borealis. 2. Stimulation of one of a set of identified serotonergic/cholinergic mechanosensory cells, the gastropyloric receptor (GPR) cells, induced plateau potentials in DG. A brief pressure application of serotonin (5-HT) closely mimicked the effect of the GPR cells. The 5-HT-evoked plateau in DG was not blocked by the sodium channel blocker, tetrodotoxin (TTX), or a combination of TTX with potassium channel blockers, including tetraethylammonium (TEA) and 4-aminopyridine (4-AP), and the Ih blocker, CsCl. The 5-HT-evoked plateau was eliminated by the Ca2+ channel blockers Co2+ and Cd2+, suggesting that Ca2+ entry is essential for plateau potentials in DG. During the plateau, we observed a 30% decrease in input resistance. 3. When sodium and potassium currents were blocked pharmacologically, injection of suprathreshold depolarizing current evoked all-or-none plateau-like responses lasting several seconds, even in the absence of 5-HT. This response was blocked by Ca2+ channel blockers, further supporting a role for Ca2+ in plateau generation. 5-HT significantly prolonged the duration of this plateau. 4. We isolated a voltage-dependent Ca2+ current in voltage-clamped DG neurons. This current was analyzed with the use of either Ca2+ or Ba2+ as the charge carrier after other currents had been maximally blocked with extracellular TTX, TEA, 4-AP, and CsCl and intracellular loading with Cs+ and ethylene glycol-bis(beta-aminoethyl ether)-N,N,N',N'-tetraacetic acid (EGTA). The Ca2+ current was detectable at -45 mV, peaked at -15 mV, and was estimated to reverse at +45 mV. Co2+ and Cd2+ effectively blocked the Ca2+ current. 5. The voltage dependence of activation of the Ca2+ current was quantantitively analyzed by fitting the voltage-conductance relation with a third power Boltzmann relation. The maximum conductance (gA), half-activation voltage (VA) for individual gating steps, and the slope steepness (k) were 0.19 +/- 0.02 (SE) microS, -36.5 +/- 2.0 mV, and 4.4 +/- 1.4 mV/e-fold, respectively. 6. 5-HT significantly potentiated the gA by approximately 42% without affecting VA and k. 7. We conclude from our current- and voltage-clamp results that a voltage-dependent Ca2+ current plays an important role in generating plateau potentials in the DG neuron. Enhancement of the voltage-dependent Ca2+ current by 5-HT is one of the mechanisms for 5-HT-evoked plateau potentials.